Arrangement for amplifying electrical oscillations



April 10, 1934. c. s. FRANKLIN ET AL ARRANGEMENT FOR AMPLIFYING ELECTRICAL OSCILLATIONS Original Filed July 16. 1925 INVENTORS CHARLES s. FRANKLIN KERNEST GREEN 4 15M TORNEY Patented Apr. 10, 1934 PATENT OFFICE ARRANGEMENT FOR AMPLIFYING ELEC- TRICAL OSCILLATIONS Charles Samuel Franklin,

St. Michaels, and

Ernest Green, Hendon, London, England, assignors to Radio Corporation of America, a corporation of Delaware Application July 16, 1925, Serial No. 43,924. Re-

newed June 15, 1931.

In Great Britain July 22 Claims. (01. 179-171) This invention relates to amplifying systems for high frequency oscillations, and is especially applicable to amplifying systems dealing with frequencies of the order of 3000 kilocycies per second or more.

In a wireless telegraph or telephonetransmitter, the large valve or power generator is now commonly controlled by a smaller generator, called the drive, and in some cases a still smaller generator, called the sub-drive, is used to control the drive, so that the output from the large valve or power generator of the amplifying system is controlled by an initial small output. In order that the frequency of the power generator in such a case may be perfectly dependent on the frequency applied by the drive, it is'necessary that there should not be any coupling between the amplifying circuits which contain the power valve and the drive circuits, and inasmuch as the capacity between the anode and grid of the power valve provides a coupling between the power circuits and the drive circuits, some anticoupling device is usually employed. This anticoupling device may be either electrostatic or electromagnetic. The aim of this anti-coupling device is to produce currents in the power circuits which give rise to two equal and opposite ing drawing, in which potentials in the grid circuit, so that the grid potential is wholly dependent upon the drive. These anti-coupling devices are quite elfective when waves several hundred meters long are being transmitted, but the adjustments become more and more difficult as the wave length is shortened, because the capacity between the electrodes of the valve becomes greater in proportion to that of the condensers employed in the circuits. Also the general electrostatic capacity of the power circuits forms part of the capacity of the grid circuits, so that any variation of it affects the tune of the grid circuit.

The object of this invention is to provide a generator in which the frequency of the oscillations generated by the power valve shall be as nearly as possible completely dependent upon the drive even when these oscillations are of very small wave length.

According to this invention the grid and anode of the power valve are placed in one arm of a bridge, the other three arms of which are formed of condensers, the capacity of each of which is effectively equal to that betweenthe anode and grid of the power valve. The power circuit is connected across the corner of the bridge which is connected to the anode and the opposite corner. The grid circuit, which is coupled to the output circuit of the drive, forms the other diagonal of the bridge, and both grid and power circuits are arranged symmetrically with respect to the earth. Preferably also this grid circuit is shunted by a high resistance some point of which is earthed in order to prevent any tendency of the system to oscillate to earth.

The two circuits are theoretically uncoupled if the capacity arms of the bridge are adjusted to give a balance according to the laws of a Wheatstone bridge. We have found that a true balance is obtained only when the grid and power circuits are symmetric with regard to the earth, as occurs when the capacities are effectively equal. For this condition the general electrostatic capacity of the power circuit has no influence on the tune of the grid circuit.

If two power valves are employed, the second may be placed in the arm of the bridge which is opposite to that in which the first is placed, the anodes of the two valves being connected to opposite corners.

Small adjustable condensers may be connected across the anodes and grids of the two valves, so that the capacities of the four arms of the bridge may be the more. readily made equal.

Our invention is illustrated in the accompanythe anode, grid and filament of a power valve 1?.

The condenser formed by A and G is arranged as one of the arms of a Wheatstone bridge,v the other arms being formed by condensers C1, C2 and'C3, which should be equal to each other and to the capacity between A and G. The resistance of the said arms should also be equal, and small adjustable resistances r1, r2, T3 and T4, are placed in the arms to ensure this. A small condenser n is also connected from the common point 3 of C1 and C2 to earth. The capacity of it should be equal to the capacity between the grid G and filament F of the valve. The primary circuitis connected across points 1 and 2 and the grid circuit across 3 and 4. Points 1 and 2 constitute a pair of conjugate neutral points of the bridge and points 3 and 4 constitute an opposite pair of conjugate neutral points. That is, when a difference of potential is impressed across. points 3 and 4 across which the grid circuit of tube Pis connected, no resulting difierence of potential between points 1 and 2 occurs, due to the balanced bridge connections. The primary circuit comprises inductances L1 and L2 and adjustable condensers K1 and K2, preferably separate from and in parallel with the condensers forming the bridge. It is possible, however, to include capacities equivalent to those of K1 and K2 in the condensers forming the bridge, and to make the inductances L1 and L2 adjustable. It is convenient to connect the positive high tension power supply at the point X between the inductances.

The grid circuitcomprises the inductance V and adjustable condensers R1 and R2 in parallel with the condensers forming the bridge. The Variable condensers of the primary and grid circuits are each shown as two condensers in series with the centre point earthed; this is desirable, but not essential.

Across the grid circuit are two resistances W1 and W2 in series, with their common point connected through a blocking condenser Z to earth and across which is provided a battery B for obtaining any desired degree of negative bias on the grid of the valve. The inductance V of the grid circuit is coupled to the output circuit of a drive oscillator (not shown). Any oscillation induced in the grid circuit produces potential variations across points 3 and 4. of the bridge. Currents flow through the bridge in two parallel paths, one through the condenser formed by G and A and the condenser C1 in series, the other through the condensers C2 and C3 in series. If the variation of potential of the points 3 and 4 is symmetric as regards the earth, and if all the condensers and resistances of the bridge arms are equal, then no variation of potential of the points 1 and 2 will occur, either with respect to each other or to earth; consequently no current can be induced in the primary circuit. It will be noticed that the whole of the part of the primary circuit comprising the inductances L1 and L2, and variable condensers K1 and K2, remains at earth potential and forms no part of the grid circuit. As a result, any adjustments made in this circuit aifect neither the balance nor the tune of the grid circuit.

Fig. 2 shows a modification in which the condensers C2 and n of Figure 1 are replaced respectively by the anode grid and grid cathode capacities of a second power valve P. In this modification, the balance condition of the Wheatstone bridge is maintained, and the employment of the two valves enables double power to be used.

It is evident that with such arrangements currents in the power circuit produce no currents in the grid circuit, and also that the tune of the grid circuits Will not be affected by earthing any part of the power circuit or by altering its electrostatic capacity with respect to earth.

A number of such amplifying circuits may be employed in cascade enabling a high power transmitter to be maintained constant in frequency by means of quite a small control oscillator. It has been found possible to control through two such systems a 25 kilowatt 25 meter transmitter by means of a 100 watt control oscillator perfectly satisfactorily.

Such a series forms in effect a high frequency amplifier and can be similarly employed in connection with the receiving of short Waves. With such a series adapted for standard receiving valves, very good and stable high frequency amplification has been obtained when receiving a wave as short as 6 meters.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:

1. A circuit arrangement for amplifying high frequency oscillations comprising a thermionic valve having an output circuit, and a control or input circuit therefor, means for preventing said input circuit from oscillating with respect to earth and means for preventing change of tuning of the control circuit by any alteration of electro-static capacity of the output circuit with respect to earth.

2. A circuit arrangement for amplifying high frequency oscillations, comprising a thermionic valve having an output circuit and an input circuit and means for preventing change of tuning of the input circuit comprising a divided impedance connected in shunt across said output circuit and a connection from an intermediate point of said impedance to ground.

3. A circuit arrangement for amplifying high frequency oscillations, comprising a thermionic valve having associated therewith a plate circuit and a grid circuit, a capacity connected across the grid circuit of said valve, two capacities in series connected across the plate circuit of said valve, the value of said capacities bearing the proper relation to each other and to the inherent capacity between the plate and grid of said valve to satisfy the conditions of a balanced Wheatstone bridge and means for compensating for the grid to filament capacity of said valve.

4. In an amplifier for high frequency oscillations including an amplifying valve having anode and grid circuits, means for avoiding reaction between the anode and grid circuits of the amplifying valve, the said means comprising a balanced Wheatstone bridge of which one arm includes the grid anode capacity of the valve, the said anode and grid circuits being connected as diagonals of the said Wheatstone bridge, and arranged symmetrically with respect to the earth so that oscillations in either circuit produce no potential in the other circuit except oscillations in the grid circuit which varies the potential of the grid relative to the cathode of the valve and equalizing resistances in the arms of said bridge.

5. In a circuit arrangement for amplifying high frequency oscillations, a Wheatstone bridge circuit having the grid and plate of a vacuum tube in one arm of the bridge, each of the other arms of said bridge having a condenser equal to each other and to the capacity between grid and plate, an adjustable resistance included in each arm for obtaining a true balance of said bridge, an input circuit connected to the grid and to the opposite corner forming one diagonal of the bridge, and an output circuit connected to the plate and to the opposite corner forming the other diagonal of the bridge, both grid and plate circuits being arranged symmetrically with respect to the earth.

6, A circuit arrangement for amplifying high frequency oscillations comprising a thermionic valve having an output circuit and an input circuit, means for maintaining said input circuit symmetrical with respect to earth, for preventing change of tuning of the input circuit by an alteration of electrostatic capacity of the output circuit, a' resistance. circuit shunted across said input circuit, a condenser having one end connected to approximately the midpoint of said resistance circuit and its other end to ground, and, a source of current shunted across said condenser.

7. An amplifying system comprising an electron discharge device having inputand output circuits, a Wheatstone bridge, one branch thereof including the plate and grid electrodes of said electron discharge device, the other branches comprising balancing means, a diagonal for said bridge comprising said input circuit, said output circuit forming the other diagonal of said bridge, a capacity connected across one end of said first mentioned diagonal and ground, and a pair of condensers in series in said input circuit and having their common point grounded.

8. An amplifying system comprising a thermionic valve having input and output circuits, a Wheatstone bridge, one branch thereof in cluding the plate and grid electrodes of said triode, the other branches comprising balancing means, a diagonal for said bridge comprising said input circuit, said output circuit forming the other diagonal of said bridge, a capacity connected across one end of said first mentioned diagonal and ground, and a pair of variable condensers in series connected with said output circuit and having their common point connected to ground.

9. An amplifying system comprising a thermionic valve having input and output circuits, a Wheatstone bridge, one branch thereof comprising grid and plate electrodes of said valve, the other branches comprising balancing devices, said input circuit being arranged to form one diagonal of said bridge and said output circuit another diagonal thereof, a pair of variable condensers in series connected with said output circuit, a second pair of variable condensers in series connected with said input circuit, and means for connecting the common point of said first mentioned pair of condensers to the common point of said second mentioned pair of condensers.

10. An amplifying system comprising a valve having input and output circuits, a Wheatstone bridge, one branch thereof comprising the. grid and plate electrodes of said valve, the other branches comprising balancing devices, said input circuit being arranged to form one diagonal of said bridge and said output circuit another diagonal thereof, a pair of variable condensers in series included in said output circuit, a second pair of variable condensers in series included in said input circuit, means for connecting the common point of said first mentioned pair of condensers to the common point of said second mentioned pair of condensers, and means in said input circuit adapted to place a negative bias on the grid of said valve.

11. An amplifying system comprising a plurality of thermionic valves having input and output circuits, a Wheatstone bridge circuit, a branch thereof including the plate and grid electrodes of one of said valves, a second branch thereof comprising the grid plate electrodes of another of said valves, the other branches comprising balancing devices, a diagonal for said bridge comprising said input circuit, said output circuit forming the other diagonal of said bridge and means comprising the grid cathode capacity of said second mentioned valve for balancing the grid cathode capacity of said first mentioned valve and means in said input circuit adapted to place a negative bias on said first mentioned valve.

12. An amplifying system comprising a valve having input and output circuits, a Wheatstone bridge circuit, a branch thereof including the plate and grid electrodes of said valve, a second branch thereof comprising the grid plate electrodes of a second valve, the other branches comprising balancing devices, a diagonal for said bridge comprising said input circuit, said output circuit forming the other diagonal of said bridge and means comprising the grid cathode capacity of said second mentioned valve for balancing the grid cathode capacity of said first mentioned triode, a resistance circuit shunted across said input circuit, a condenser having one end connected intermediate said resistance circuit, and its other end to ground and a source of current shunted across said condenser.

13. An amplifying system comprising a valve having input and output circuits, a Wheatstone bridge circuit, a branch thereof including the plate and grid electrodes of said valve, a second branch thereof comprising the grid plate electrodes of a second valve, the other branches comprising balancing devices, a diagonal for said bridge including said input circuit, said output circuit forming the other diagonal of said bridge, means comprising the grid cathode capacity of said second mentioned valve for balancing the grid cathode capacity of said first mentioned valve, and a pair of condensers in series connected with said input circuit and having their common point grounded.

14. An amplifying system comprising a valve having input and output circuits, a Wheatstone bridge circuit, a branch thereof including the plate and grid electrodes of said valve, a second branch thereof comprising the grid plate electrodes of a second valve, the other branches comprising balancing devices, a diagonal for said bridge including said input circuit, said output circuit forming the other diagonal of said bridge, means comprising the grid cathode capacity of said second mentioned valve for balancing the grid cathode capacity of said first mentioned valve, and, a pair of variable condensers in series in said output circuit and having their common point connected to ground.

15. An amplifying system comprising a valve having input and output circuits, a Wheatstone bridge circuit, a branch thereof including the plate and grid electrodes of said valve, a second branch thereof comprising the grid plate electrodes of a second valve, the other branches comprising balancing devices, a diagonal for said bridge including said input circuit, said output circuit forming the other diagonal of said bridge, means comprising the grid cathode capacity of said second mentioned valve for balancing the grid cathode capacity of said first mentioned valve, a pair of condensers in series shunted across said output circuit, a second pair of condensers in series shunted across said input circuit, and means for connecting the common point of said first mentioned pair of condensers to the common point of said second mentioned pair of condensers.

16. A circuit arrangement for high frequency oscillations comprising an electron discharge device having an input circuit and an output circuit, a bridge circuit having in one of its arms, the interelectrode capacity between a plurality of electrodes of said electron discharge device, the

other arms of said bridge comprising balancing said input circuit is symmetrical with respect to ground for high frequency currents.

17. A circuit arrangement for high frequency oscillations comprising an electron discharge device having an input circuit and an output cir cuit, a bridge circuit having in one of its arms, the interelectrode capacity between a plurality of electrodes of said electron discharge device, the other arms of said bridge comprising balancing elements, means for connecting said input circuit to conjugate points on said bridge, means connecting an output circuit to conjugate points on said bridge, and variable capacities connecting similar points on said output circuit to ground whereby said output circuit is symmetrical with respect to ground for high frequency currents.

18. A circuit arrangement for high frequency oscillations comprising an electron discharge device having an input circuit and an output cir cuit, a bridge circuit having in one of its arms, the interelectrode capacity between a plurality of electrodes of said electron discharge device, the other arms of said bridge comprising balancing elements, means for connecting said input circuit to conjugate points on said bridge, means connecting said output circuit to conjugate points on said bridge, means for connecting similar points on said input circuit to ground whereby said input circuit is symmetrical With respect to ground for high frequency currents, and, means for connecting similar points on said output circuit to ground whereby said output circuit is symmetrical with respect to ground for high frequency currents.

19. In apparatus for high frequency undulatory electrical currents, a pair of electron discharge devices, an input circuit for said devices, capacitive means for connecting similar points on said input circuit to ground whereby said input circuit is symmetrical with respect to ground for high frequency currents, an output circuit for said electron discharge devices, and, a bridge circuit, said bridge circuit having in opposite arms interelectrode capacities of said electron discharge devices in other opposite arms balancing elements, said input circuit being connected across conjugate points on said bridge circuit and said output circuit being connected across other conjugate points on said bridge circuit.

20. In apparatus for high frequency undulatory electrical currents, a pair of electron discharge devices, an input circuit for said devices, parallel capacitive and resistive means for connecting similar points on said input circuit to ground whereby said input circuit is symmetrical with respect to ground for high frequency currents, an output circuit for said electron discharge devices, impedances for connecting points on said output circuit to ground whereby said output circuit is rendered symmetrical with respect to earth for high frequency currents, and, a bridge circuit, said bridge circuit having in opposite arms interelectrode capacities of said electron discharge devices in other opposite arms balancing elements, said input circuit being connected across conjugate points on said bridge circuit and said output circuit being connected across other conjugate points on said bridge circuit.

21. In a radio frequency apparatus, means for balancing the inner electrode capacities of two audions, consisting of two audions disposed in two branches of a Wheatstone bridge, a pair of separate plate windings also disposed in said branches of said Wheatstone bridge, balanced impedances disposed in the other two branches of said Wheatstone bridge, an input circuit constituting one diagonal of said bridge, and sources of current, and filaments of said audions constituting the other diagonals of said bridge.

22 A system including the four arms and the diagonals of a Wheatstone bridge, an input circuit of which one diagonal of said bridge is an element, two audions and their plate windings arranged in adjacent arms of said bridge, and the other two arms of said bridge constituting similar serial impedances connected across said input circuit, and a biasing source of energy disposed in the other diagonal of said bridge.

CHARLES SAMUEL FRANKLIN. ERNEST GREEN. 

